SuperGraphs for the Sinclair ZX-81 Computer

Help Wanted - ZX-81 Output resists Capture!

October 2006

This page is intended to present the development of SuperGraphs,
a charting application written in assembly language for the tiny Sinclair ZX-81 computer
of the early 1980s. The original SuperGraphs Users Guide in Acrobat is
available here

To see a larger picture, click any image

We intended to use a digital video capture card to capture the animated demo for SuperGraphs, played on the ZX-81,to a video
available on this page. After a month of excellent input from some of the great
minds on the Web, it appears that the only way to get the funky ZX-81 output
to be recognized by a digital video capture card is to use a Time Base
Corrector. Alas, access to such a device has proven daunting.

As a result, the demo has been made Blair Witch style, using a Canon Elura
100 Camcorder on a tripod, pointed at the flat screen TV screen, recorded to a
Mini-DV tape in the camera, firewired into the computer to a file, and mixed
with the original audio soundtrack using Windows Movie Maker.

There is a black band that rolls through the video caused by the sync
differences. Think of this as "artistic dramatic
enhancement". :-)

After 20 years in the garage, the old ZX-81 with it's trusty 64k
MemoPak and QSave hardware was connected to an old 12" black and
white TV through it's TV channel 3 RF interface, and an old SuperScope cassette
deck.

After a bit of fussing and cassette head cleaning, the ZX-81 booted up
and was running SuperGraphs once again!

Ready to start the video capture, the TV channel 3 RF interface from the
ZX-81 was connected to the Leadtek WinFast TV2000 XP Expert Composite input.
After initially showing just a few frames of the computer output, the captured video faded to black. The capture card
is in a 2.8Ghz P4 Intel D865PERL motherboard running Windows XP in 1G memory
with 760 GB storage in 3 drives. Here is the AV
Connections diagram for the system.

The RF inputs of several old VCRs connected to TVs were tried with the same results.
However, when connected to the RF input of a Daewoo DTQ27U4SC 27" flat screen
TV, the ZX-81
picture was perfect!

The Daewoo 27" TV has a composite video monitor out jack that provides an output signal
for the built in TV tuner and all composite video sources connected to the back
of the TV. We connected that monitor out
jack directly to the WinFast capture card composite input and also tried it through a
Jensen ISO-MAX video isolation
transformer.

Whenever any video from the Daewoo not from the ZX-81 was
selected for viewing and monitor output, the WinFast card captured it perfectly!
Using or not using the ISO-MAX made no difference in any of the results.

Here is a screen cap from VirtualDub
showing a normal TV signal and its
associated histogram from the Daewoo monitor jack being captured by the WinFast
card being monitored in VirtualDub's capture mode:

Here is a screen cap from VirtualDub showing the ZX-81 signal and its
associated histogram from the Daewoo monitor jack being captured by the WinFast
card being monitored in VirtualDub's capture mode:

Click here to see a Windows Media video
of what is seen through the capture card in the first second when the Daewoo is
switched to the ZX-81 signal. After initially showing just a few frames of
the computer output, the captured video fades to black.

Finally, here is a screen cap of one frame of that captured video, when a ZX-81
SuperGraphs menu screen appears in pretty good shape!

We tried every filter and setting in both VirtualDub's capture interface and in
dScaler's capture interface.
Absolutely nothing made any difference.

We are hoping an expert out there can tell us why the ZX-81
signal can be seen on a couple TVs, yet the 75 ohm composite out of this TV image
cannot be seen on a capture card except for a few frames.